Tens of thousands of patients are expected to quickly take advantage of medical marijuana in Massachusetts, where the first dispensaries are on pace to open this summer and provisional licensees are all expected to make home deliveries, according to the state official in charge of the program’s launch.

During a presentation to a rapt audience of mayors, selectmen and school officials Tuesday, Karen van Unen, executive director of the Department of Public Health medical marijuana program, said it has taken off like “rapid lightning” since voters last year passed a ballot law directing the state to embrace medical marijuana.

About 120,000 patients are expected to use medical marijuana over the program’s first two years, van Unen told local officials. She told the News Service after her presentation that the estimate is based on the experience in Colorado with its medical marijuana program.

For comparison’s sake, 120,000 people roughly equals the size of a large Massachusetts city, more than the population of Lowell but lower than the size of Springfield.

“This is definitely a brave new world for Massachusetts,” said Salem Mayor Kim Driscoll, whose city is scheduled to host a dispensary.

Driscoll said she appreciated the “serious-minded nature” of the state’s licensing process and hoped the department would make personnel available for one-on-one communications over local issues.

Van Unen, the former chief operating officer of a Dorchester public health program, said she was “absolutely delighted” with the process to date, calling it “multi-layered” and “robust.”

Critics have knocked the licensing process as too secretive and raised questions about Public Health Commissioner Cheryl Bartlett’s relationship with William Delahunt, a former Congressman who is part of a team that secured three licenses, as well as van Unen’s appointment late in the licensing process.

Republican Party officials last month ripped the DPH process and called on Gov. Deval Patrick, who has been coping with major administrative shortcomings in recent months, to hand over medical marijuana licensing to an independent commission. At the time, a state health and human services secretariat spokesman declined comment, saying the administration does not respond to the GOP.

While licensees are permitted to grow only enough marijuana to serve their patients, state rules allow patients to be served at any dispensary in the state.

“We really wanted to ensure that patients had choice,” said van Unen, who added that the state encouraged dispensaries to offer home delivery to patients and all approved applicants plan to do so.

Dispensaries are allowed to grow marijuana at locations apart from their retail locations, she said.

Page 2 of 2 - State officials are setting up one-on-one meetings with each approved applicant to explain inspectional processes, security procedures and product testing policies and steps they need to comply with before receiving final certificates of registration. Provisional licensees are also being urged to connect with local officials and boards of health to work through zoning, permitting and issues surrounding expectations.

The voter law requires at least one dispensary in every county. Four counties - Martha’s Vineyard, Nantucket, Berkshire and Franklin - do not currently have an approved provisional licensee, although eight applicants are in the running to serve patients in those areas.

On Friday, those eight applicants were invited by state officials to update their applications, with a focus placed on siting. Applicants have until early May to complete updates, van Unen said, and announcements about a second round of provisional licensees are expected in early June.

“That will be kind of the next wave,” said van Unen, who urged local officials in communities that may host dispensaries to have conversations with applicants about potential locations.

“It’s up to them and it’s obviously up to the municipalities in terms of the potential match,” van Unen said.

Middlesex County on Jan. 31 received the highest number of dispensary licenses, with one each in Lowell, Ayer, Newton and Cambridge.